Time Resolved

We report some time-resolved luminescence measurements on the conjugated polymer, MEH-PPV, that show a marked dependence on the intensity of the excitation light. At low laser powers the excited state lifetime is longer than has been reported previously and is consistent with emission from an interchain excitation that has low oscillator strength and hence a long natural radiative lifetime. We rationalise these data with a simple scheme that summarises the excited state kinetics in this and other conjugated, electroluminescent polymers.

Heterocyclic ligands are common sensitizers for the lanthanide ions emission upon UV irradiation. Of particular interest are Tb(III) and Eu(III) ions in combination with acyclic, macrocyclic and macropolycylic ligands incorporating heterobiaryls, such as 2,29-bipyridyl, 1,10-phenanthroline, 1,19-biquinoline, 3,39-biisoquinoline, and their derivatives. Much more powerful sensitizers are the same systems bearing N-oxide functions, furthermore their complexes are more stable than the parent heterocycles. Such luminescent materials are the basis for time-resolved fluoroimmunoassays displaying very high sensitivity, and serve as materials for advanced medical diagnostics. In this short review, a brief presentation will focus on state-of-the-art photochemistry and photophysics of the lanthanide complexes of heterocyclic and N-oxide-incorporating heterocyclic systems.

The impact of intra-ensemble Förster resonant energy transfer (FRET) on the optical properties of monodispersed quantum dot (QD) monolayers and a donor/acceptor FRET bilayer structure are presented. The QD structures are characterized by steady-state absorption and photoluminescence (PL) spectroscopy as well as time-resolved PL measurements. The optical properties of the monodispersed monolayers, such as peak emission wavelength and PL decays, are strongly influenced by FRET from smaller to larger QDs within the ensemble. Comparing several QD samples, the spectral overlap of the QD ensemble and the QD concentration were identified as parameters that allow for tuning of FRET in monodispersed QD structures. For the donor/acceptor QD bilayer structure an unexpected decrease of the FRET efficiency between donor and acceptor layers is observed with increasing donor QD concentration. The concentration-dependent donor lifetime and a constant donor-acceptor FRET rate can explain this decrea...

The mechanical behavior of human hair fibers is determined by the interactions between keratin proteins structured into microfibrils (hard ␣-keratin intermediate filaments), a protein sulfur-rich matrix (intermediate filaments associated proteins), and water molecules. The structure of the microfibril-matrix assembly has already been fully characterized using electron microscopy and small-angle x-ray scattering on unstressed fibers. However, these results give only a static image of this assembly. To observe and characterize the deformation of the microfibrils and of the matrix, we have carried out time-resolved small-angle x-ray microdiffraction experiments on human hair fibers stretched at 45% relative humidity and in water. Three structural parameters were monitored and quantified: the 6.7-nm meridian arc, which is related to an axial separation between groups of molecules along the microfibrils, the microfibril's radius, and the packing distance between microfibrils. Using a surface lattice model of the microfibril, we have described its deformation as a combination of a sliding process and a molecular stretching process. The radial contraction of the matrix is also emphasized, reinforcing the hydrophilic gel nature hypothesis.

... Available online 12 March 2002. Abstract. The present contribution shows the results of tests on geological materials for determination of the rare-earth elements La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, as well as Y and Sc. ...

When redox active molecules are covalently tethered to the surface of the n-doped silicon, light induces their oxidation by the semiconductor. To visualize this charge separation we have followed the formation and decay of the surface photovoltage by an auxiliary transparent electrode, which serves as a capacitive probe that picks up the transient surface photovoltage. This method has been used in electronics and biophysics but is novel for the surface redox chemistry. The probe provides both the direction of the charge transfer and the resolution of the dynamics. Its application is independent of the optical properties of the semiconductor. Laser initiation of the reaction permits quantitative measurements of the reactions with the rate constants ~108-103 s-1. The electron injection from covalently tethered NTCI(A) molecules into silicon was fast (k >109 s-1). The back electron transfer is poly-disperse with the components ranging from ~10 microseconds and up.

Hydrogen bonding substrates (S) trigger bright phosphorescence upon their addition to aqueous solutions containing 1bromonaphthalene (1-BrNp) and a glucosyl modified eyclodextrin (C~-CD). Steady-state and time-resolved luminescence measurements establish that the phosphorescence arises from 1-BrNp as part of a 1-BrNp.G/1-CD.S complex. In all cases, the substrate comprises a bulky t-butyl or cyclohexyl group spaced from the functional group of an alcohol, amine, carboxylie acid, ester, or aldehyde. Association constants, molecular models, and substrate binding studies are consistent with the substrate hydrogen bonding to the rim of the CD cup, with its aliphatic end flipped over the hydrophobic interior of the CD. The phosphorescence enhancement induced by substrate is related to its effectiveness in shielding photoexcited 1-BrNp from quenching by oxygen. The enhancements can be extremely large, approaching a 10 5 increase in intensity, depending on the fit of the substrate to the top of the CD. We have explored the topological features of the substrate that lead to the best shielding and hence greatest triggered luminescence response.

Acetohydroxy acid synthase (AHAS) and related enzymes catalyze the production of chiral compounds [(S)-acetolactate, (S)-acetohydroxybutyrate, or (R)-phenylacetylcarbinol] from achiral substrates (pyruvate, 2-ketobutyrate, or benzaldehyde). The common methods for the determination of AHAS activity have shortcomings. The colorimetric method for detection of acyloins formed from the products is tedious and does not allow time-resolved measurements. The continuous assay for consumption of pyruvate based on its absorbance at 333 nm, though convenient, is limited by the extremely small extinction coefficient of pyruvate, which results in a low signal-to-noise ratio and sensitivity to interfering absorbing compounds. Here, we report the use of circular dichroism spectroscopy for monitoring AHAS activity. This method, which exploits the optical activity of reaction products, displays a high signal-to-noise ratio and is easy to perform both in time-resolved and in commercial modes. In addition to AHAS, we examined the determination of activity of glyoxylate carboligase. This enzyme catalyzes the condensation of two molecules of glyoxylate to chiral tartronic acid semialdehyde. The use of circular dichroism also identifies the product of glyoxylate carboligase as being in the (R) configuration.

We present a new bidimensional detector setup, based on cross delay line technology, specifically developed for time resolved experiments and particularly suited to work in conjunction with pumpand-probe systems. Thanks to the particular architecture of the acquisition electronics, the detector is able to correlate each event with the time it occurred in a way which preserves the picoseconds time resolution of pump-and-probe techniques and, more generally, can perform time resolved acquisition in the nanosecond or picoseconds scale. The acquisition setup count rate, up to more than 4 Mcounts/s in time resolved mode, exceeds the performances of the best two-dimensional detectors working in counting mode presently available on electron analysers.

We report grazing incidence coherent X-ray measurements from polymer bilayers consisting of spun-cast layers of Polystyrene (PS) and Poly(4-bromo styrene) (PBrS) supported on silicon wafers. For PS/PBrS/Si bilayers, the films are stable and we are able to probe equilibrium thermal surface height fluctuations using X-ray Photon Correlation Spectroscopy (XPCS). When the layers are inverted to PBrS/PS/Si, the films de-wet. In this geometry we can measure both the non-equilibrium evolution of the film structure using timeresolved surface diffuse X-ray scattering and quasi-equilibrium fluctuations of the de-wetting film using XPCS.

Time-resolved small-angle X-ray and neutron scattering (SAXS and SANS) in solution were used to study the swelling reaction of TBSV upon chelation of its constituent calcium at mildly basic pH. SAXS intensities comprise contribution from the protein capsid and the RNA moiety, while neutron scattering, recorded in 72% D 2 O, is essentially due to the protein capsid. Cryo-electron micrographs of compact and swollen virus were used to produce 3D reconstructions of the initial and final conformations of the virus at a resolution of 13 Å and 19 Å , respectively. While compact particles appear to be very homogeneous in size, solutions of swollen particles exhibit some size heterogeneity. A procedure has been developed to compute the SAXS pattern from the 3D reconstruction for comparison with experimental data. Cryoelectron microscopy thereby provides an invaluable starting (and ending) point for the analysis of the time-resolved swelling process using the scattering data.

The activation of complement via the mannan-binding lectin (MBL) pathway is initiated by the MBL complex consisting of the carbohydrate binding molecule, MBL, two associated serine proteases, MASP-1 and MASP-2, and a third protein, MAp19. In the present report we used an assay of complement activation specifically reflecting the physiological activity of the MBL complex to identify biological and synthetic inhibitors. Inhibitor activity towards the MBL complex was compared to the inhibition of the classical pathway C1 complex and to a complex of MBL and recombinant MASP-2. A number of synthetic inhibitors were found to differ in their activities towards complement activation via the MBL pathway and the classical pathway. C1 inhibitor inhibited both pathways whereas a 2 -macroglobulin (a 2 M) inhibited neither. C1 inhibitor and a 2 M were found to be associated with the MBL complex. Upon incubation at 37°C in physiological buffer, the associated inhibitors as well as MASP-1, MASP-2, and MAp19 dissociated from MBL, whereas only little dissociation of the complex occurred in buffer with high ionic strength (1 M NaCl). The difference in sensitivity to various inhibitors and the influence of high ionic strength on the complexes indicate that the activation and control of the MBL pathway differ from that of the classical pathway. MBL deficiency is linked to various clinical manifestations such as recurrent infections, severe diarrhoea, and recurrent miscarriage. On the other hand, impaired control of complement activation may lead to severe and often chronically disabling diseases. The results in the present report suggests the possibility of specifically inhibiting of the MBL pathway of complement activation.

Semiconducting single-walled carbon nanotubes (SWCNTs) sorted by conjugated polymers are of great interest for electronic and optoelectronic applications. Here we demonstrate by optical methods that the selectivity of conjugated polymers for semiconducting SWCNTs is influenced by the structure of their side-chains and/or the molecular weight of the macromolecules, and that side chain functionalities determine the solubility in different dispersion media. Moreover, high resolution time-resolved photoluminescence measurements provide evidence of energy transfer from tubes with larger band gaps compared to those with smaller band gaps coexisting in SWCNT bundles. address: M.A.Loi@rug.nl (M.A. Loi). C A R B O N 4 9 ( 2 0 1 1 ) 3 3 3 -3 3 8 a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c a r b o n

Oxygen minimum zones (OMZs), also known as oceanic "dead zones", are widespread oceanographic features currently expanding due to global warming and coastal eutrophication. Although inhospitable to metazoan life, OMZs support a thriving but cryptic microbiota whose combined metabolic activity is intimately connected to nutrient and trace gas cycling within the global ocean. Here we report time-resolved metagenomic analyses of a ubiquitous and abundant but uncultivated OMZ microbe (SUP05) closely related to chemoautotrophic gill symbionts of deep-sea clams and mussels. The SUP05 metagenome harbors a versatile repertoire of genes mediating autotrophic carbon assimilation, sulfur-oxidation and nitrate respiration responsive to a wide range of water column redox states. Thus, SUP05 plays integral roles in shaping nutrient and energy flow within oxygen-deficient oceanic waters via carbon sequestration, sulfide detoxification and biological nitrogen loss with important implications for marine productivity and atmospheric greenhouse control.

Ultrasensitive, near-infrared (NIR), time-resolved fluorescence is evaluated as a detection method for reading DNA hybridization events on solid surfaces for microarray applications. In addition, the potential of mulitiplexed analyses using time-resolved identification protocols is described. To carry out this work, a NIR time-resolved confocal imager was constructed to read fluorescence signatures from the arrays. The device utilized a 780-nm pulsed diode laser, a single-photon avalanche diode (SPAD), and a high-numerical-aperture microscope objective mounted in an epi-illumination format. Due to the small size of the components that are required to construct this imager, the entire detector could easily be mounted on high-resolution translational stages and scanned over the stationary arrays. The instrument response function of the device was determined to be 275 ps (fwhm), which is adequate for measuring fluorophores with subnanosecond lifetimes. To characterize the system, NIR dyes were deposited directly on different substrate materials typically used for DNA microarrays, and the fluorescence lifetimes of two representative dyes were measured. The fluorescence lifetime for aluminum tetrasulfonated naphthalocyanine was found to be 1.92 ns, and a value of 1.21 ns was determined for the tricarbocyanine dye, IRD800, when it was deposited onto poly(methyl methacrylate) (PMMA) and measured in the dry state. Finally, the imager was used to monitor hybridization events using probe oligonucleotides chemically tethered to a PMMA substrate via a glutardialdehyde linkage to an aminated-PMMA surface. The limit of detection for oligonucleotides containing a NIR fluorescent reporter was determined to be 0.38 molecules/µm 2 , with this detection limit improving by a factor of 10 when a time-gate was implemented. Fluorescence lifetime analysis of the hybridization events on PMMA indicated a lifetime value of 1.23 ns for the NIR-labeled oligonucleotides when using maximum-likelihood estimators.

We introduce a new scheme for investigating temporally heterogeneous dynamics, which is termed time-resolved correlation (TRC). TRC is applied to data obtained by diffusing wave spectroscopy probing the slow dynamics of a strongly aggregated colloidal gel. Other examples of TRC data, collected for different jammed materials in single and multiple scattering, are provided to demonstrate the wide range of applicability of this method. In all cases we find evidence that the slow dynamics results from a series of discrete steps rather than from a continuous motion, suggesting temporal heterogeneities to be a general feature of slow dynamics in jammed systems.

Transient linear reflectivities of thin gold films in the thickness range from 10 nm to 500 nm have been measured by femtosecond time-resolved pump-probe experiments. A conspicuous change of the relaxation behavior was found around 100 nm for pump pulse fluences of 1 m J / cm 2 . Thicker films show a nearly exponential decay of the transient linear reflectivity, which turns into a linear decay during the first

The photosynthetic apparatus of green sulfur bacteria, the chlorosome, is generally considered as a highly efficient natural light-harvesting system. The efficient exciton transport through chlorosomes toward the reaction centers originates from self-assembly of the bacteriochlorophyll molecules. The aim of the present work is to realize a long exciton diffusion length in an artificial light-harvesting system using the concept of self-assembled natural chlorosomal chromophores. The ability to transport excitons is studied for porphyrin derivatives with different tendencies to form molecular stacks by self-assembly. A porphyrin derivative denoted as ZnOP, containing methoxymethyl substituents ({meso-tetrakis[3,5-bis(methoxymethyl)phenyl]porphyrinato}zinc(II)) is found to form self-assembled stacks, in contrast to a derivative with tertbutyl substituents, ZnBuP ({meso-tetrakis[3,5-bis(tert-butyl)phenyl]porphyrinato}zinc(II)). Exciton transport and dissociation in a bilayer of these porphyrin derivatives and TiO2 are studied using the time-resolved microwave conductivity (TRMC) method. For ZnOP layers it is found that excitons undergo diffusive motion between the self-assembled stacks, with the exciton diffusion length being as long as 15 (1 nm, which is comparable to that in natural chlorosomes. For ZnBuP a considerably shorter exciton diffusion length of 3 (1 nm is found. Combining these exciton diffusion lengths with exciton lifetimes of 160 ps for ZnOP and 74 ps for ZnBuP yields exciton diffusion coefficients equal to 1.4 × 10-6 m 2 /s and 1 × 10-7 m 2 /s, respectively. The larger exciton diffusion coefficient for ZnOP originates from a strong excitonic coupling for interstack energy transfer. The findings show that energy transfer is strongly affected by the molecular organization. The efficient interstack energy transfer shows promising prospects for application of such self-assembled porphyrins in optoelectronics.

Structure and dynamics of adenosine loops in RNA bulge duplexes was studied using time-resolved spectrofluorimetry and in aqua simulation of molecular dynamics. Thermodynamics revealed that 2-aminopurine riboside is an non-invasive fluorescent probe when built within the bulge region of chemically synthesized RNA duplexes.

RT-2 instruments will cover the energy range of 15 keV to 150 keV extendable up to ∼1 MeV. It consists of three detectors (two Phoswich detectors, namely, RT-2/S, RT-2/G and one solidstate imaging detector RT-2/CZT) and one processing electronic device (RT-2/E). Both Phoswich detectors will have time resolved spectrum, whereas the solid-state imaging detector will have high resolved image of the solar flares in hard X-rays. We have used Co-57 (122 keV) radio-active source for onboard calibration of all three detectors. In this paper, we briefly discuss the in-flight performance of RT-2 instruments and present initial flight data from the instruments.

We present a 3D implementation of the UCL imaging package for absorption and scatter reconstruction from time-resolved data (TOAST), embedded in the SCIRun interactive simulation and visualization package developed at the University of Utah. SCIRun is a scientific programming environment that allows the interactive donstruction, debugging, and steering of large-scale scientific computations. Using this "computational workbench," a scientist can design and modify simulations interactively via a dataflow programming model. SCIRun enables scientists to design and modify model geometry, interactively change simulation parameters and boundary condititions, and interactively visualize geometric models and simulation results. Reconstruction of a segmented 3D head model is used as an example for demonstrating the capabilities of the combined TOAST/SCIRun approach.

We have used high speed streaking photographic techniques to time-resolve the luminous components of cloud-to-ground lightning flashes. All recordings were made during our participation in the Thunderstorm Research International Program (TRIP), conducted at the Kennedy Space Center, Florida, during the summers of 1977 and 1978, and at the Langmuir Laboratory, near Socorro, New Mexico, during the summer of 1979. Twenty one dart leaders, four dart-stepped leaders and three stepped leaders were recorded, the majority under daylight conditions. The mean two-dimensional propagation speed of the dart leaders, evaluated over channel lengths less than or equal to 0.8 km above ground, is 11 x 10 6 m/s, with a range of 2.9 to 23 x 10 6 m/s. Several of the dart leaders reveal a decrease in propagation speed as ground is approached. However, four of the dart leaders in two separate flashes show an increase in speed near the ground, an observation not previously reported in the literature. In two multistroke flashes, we examine the variation of dart leader propagation speed along the channel and find very similar behavior for different strokes in the same flash. The speed variations that we observe may be predominantly caused by geometrical variations of the channel. The dart leader propagation speeds reported in this study are compared with the earlier works of Schonland, McEachron, and Kitagawa and Brook. Agreement among the studies is good, with a common range of observed dart leader propagation speed of 2 to 23 x 106 m/s. The major discrepancy among these studies is the observation, by Schonland, of a distribution of dart leader propagation speeds positively skewed toward the lower limit of reported values. Eleven of the dart leaders are analyzed at upper and lower levels along the visible channel to give 22 dart 'lengths.' They range from 7 to 75 m with a mean of 34 m. For these 22 determinations, we calculate a correlation coefficient of 0.85 between the dart length and the dart leader propagation speed. The correlation of greater dart length with higher propagation speed is consistent with the slower decay of channel luminosity due to the greater initial input of energy to the channel by the faster and, presumably, more energetic dart leader. Four dart-stepped leaders are examined in detail with regard to variation of propagation speed, step length, stepping interval, and luminous intensity during propagation between the cloud base and ground. Significant differences in the tendencies of these parameters are found within these four leaders. For example, one dart-stepped leader recording shows a decreasing propagation speed and an increasing step interval near ground, whereas another shows the opposite behavior. In the best event recorded, several of the individual steps reveal a photographic film density structure, with the lower portion of the step exhibiting a distinct, bright tip that fans out into a symmetrically diffuse image in the upper portion of the step. Our analysis indicates that this spread in the upper portion of the step image is not the result of streaking photography distortion but, rather, represents the luminous structure of the step. We estimate that the step image is recorded in less than 1/xs. Consequently, with a measured step length of---20 m, the luminous pulse must propagate along the step at a speed of at least 2 x 10 7 m/s. The mean propagation speed for three stepped leaders is found to be 1.1 x 106 m/s. All three stepped leaders are very faint, and were recorded only in the last 100-200 m above ground. Two stepped leaders and one dart-stepped leader do not propagate completely to ground before initiation of the return stroke. Apparently, these leaders are met by an upward propagating discharge at heights above ground of 20, 30, and 20 m, respectively. Other stepped and dart-stepped leader cases are indeterminate because an obstacle or the horizon prevent the recording of the leaders near the ground. Connecting discharges are not observed for any of the dart leader events with a resolution of 10 m at 5 km, implying that upward discharges initiated by the approach of dart leader do not occur or are substantially less than a few tens of meters in length. Dart leaders, apparently, propagate completely to ground.

A simple apparatus for time-correla ted single photon counting (TCSPC) m easurements in the near-infrared (near-IR) region for scanning-type applications has been constructed and examined. The apparatus consisted of ® ve m ajor com ponents including a pulsed diode laser source (lasing wavelength 5 780 nm; rep etition rate 5 80 MHz; power 5 5 mW ; pulse width 5 150 ps), an integrated m icroscope, a large-photoactive-area avalanche photodiode (APD), a TCSPC PC-board including the electron ics, and a Windows-based software package for accu mulating the¯uorescen ce decay pro® les. The instrument response function (IRF) of this assembly was found to be 460 ps, which is adequate for measuring lifetim es with t f $ 500 ps. Due to the sm all size of the device, it also allowed implem entation into scanning experim ents where lifetimes were measured. To dem onstrate this capability, we scanned a three-well microscope slide containing a near-IR dye. The decay pro® le of the near-IR dye, aluminum 2,3-naphthalocyanine, was collected and analyzed to obtain its lifetime, which was found to be 2.73 ns, in close agreement with the literature value for this particular dye. In addition, a three-dim ensional plot showing the decay pro® les (tim e vs. photocounts) and scan position of aluminum 2,3-naphthalocyaninē uorescence was acquired by scanning the m icroscope head over this three-well glass slide. In the scanning m ode, the IRFs as well as the decays of the dyes were found to be very stable. The device dem onstrated a concentration detection sensitivity of 0.44 nM; however, the dynamic range was limited due to the slow tim e constant (passive quenching) associated with the APD.

We report the discovery of multiperiodic luminosity variations in the hot B subdwarf KPD 1930]2752. This star was selected as a potential target in the course of our ongoing survey to search for pulsators of the EC 14026 type. Our model atmosphere analysis of the time-averaged optical spectrum of KPD 1930]2752 indicates that this star has K and log g^5.61, which places it well T eff^3 3,280 within the theoretical EC 14026 instability strip. At least 44 harmonic oscillations are seen in the light curve, with periods in the range 145È332 s, and amplitudes in the range 0.064%È0.451% of the mean brightness of the star. In addition, the light curve is dominated by a nearly sinusoidal variation with a period of D4108.9 s and amplitude of D1.39%. This latter variation is unique among the known EC 14026 stars. We argue that this relatively slow luminosity variation is likely due to the ellipsoidal deformation of the sdB star in a close binary system containing a faint invisible companion (possibly a white dwarf). Using a new period-matching technique based on a genetic algorithm, we demonstrate that the dense observed period spectrum in the 145È332 s interval is compatible with a theoretical low-degree p-mode spectrum that is rotationally split in a star rotating with a period of D8217.8 s, the value expected from the ellipsoidal e †ect invoked to explain the observed long-period variation. This interpretation awaits the test of time-resolved spectroscopy. If conÐrmed, the potential of KPD 1930]2752 as a laboratory for EC 14026 seismology will become immense.

A technique for time resolved determination of tuning rate and line width is proposed which allows online system control for tunable diode laser spectroscopy, a proven tool for scientific gas analysis below part per billion concentrations. With this innovative feature, the laser systems have improved potential for industrial process control and may become a secondary standard for example for accurate partial pressure measurements by exact determination of a gas absorption profile, minimizing the need for calibration gases. The demonstrated technique makes use of delayed selfheterodyning. Ó (R. Brunner).

We report time-resolved photoluminescence (PL) spectroscopy study of solutions and thin films of pure penta-fluorenes with varying alkyl side-chain structures. While no low-energy green emissions were observed in solutions, spin-cast and vacuum dried initial films exhibited low-energy emission in timedelayed PL spectra, centered at 490 nm, whose intensity varied strongly with the side-chain structure. In comparison, the films annealed at 170 C in air showed nearly identical low-energy emission in timedelayed PL spectra, centered at 520 nm, regardless of the side-chain structures. Therefore, our experimental results obtained for pure penta-fluorenes provided clear evidence that the mechanism of the green emission of oligofluorene and polyfluorene films includes both the intermolecular aggregate/excimer formation, emitted around 490 nm, and fluorenone-type chemical defects, emitted around 520 nm.

The fluorescence properties of Alexa 488, Oregon Green 488, and Oregon Green 514 (Molecular Probes (Eugene, OR)) are compared when conjugated to biomolecules and as model compounds free in solution. We show that these relatively new, green fluorescence probes are excellent probes for investigation of the thermodynamics of protein-protein and protein-nucleic acid interactions by fluorescence anisotropy. Unlike fluorescein, the emission of these dyes has minimal pH dependence near neutrality and is significantly less susceptible to photobleaching. Steady-state and time-resolved fluorescence anisotropy data are compared for two interacting proteins of different size and for the association of a transcription factor with a DNA oligonucleotide containing a specific binding site. The temperature dependence of the fluorescence lifetimes of the probes is reported, and the effects of molecular size and probe motion on steady-state anisotropy data are discussed. The critical interplay among correlation time, fluorescence lifetime, and the observed steady-state anisotropy is evaluated. Ó

A conceptual analytical framework was developed for the risk-based estimation of climate change effects on mine waste runoff water quality. The modeling approach incorporates temporal variability in both precipitation and temperature using trend analyses from historical datasets and the resulting effects on water balances and geochemical weathering rates. A case-study method was used to develop and present the model for regions near the city of Yellowknife, Northwest Territories, Canada. Timeresolved relative precipitation and weathering factors can be calculated using climate data and site-specific estimates regarding mine waste properties. Interpretation of these factors allows an assessment of when the period of highest analyte concentrations are expected to occur under a given risk scenario over a modeling timeframe, whether low precipitation or warming effects are more important in determining water quality issues, and the relative magnitudes of water quality under differing risk scenarios.

Singular value decomposition (SVD) separates timedependent crystallographic data into time-independent and time-dependent components. Procedures for the effective application of SVD to time-resolved macromolecular crystallographic data have yet to be explored systematically. Here, the applicability of SVD to experimental crystallographic data is tested by analyzing 30 time-resolved Laue data sets spanning a time range of nanoseconds to milliseconds through the photocycle of the E46Q mutant of photoactive yellow protein. The data contain random and substantial systematic errors, the latter largely arising from crystal-to-crystal variation. The signal-to-noise ratio of weighted difference electron-density maps is signi®cantly improved by the SVD attening procedure. Application of SVD to these¯attened maps spreads the signal across many of the 30 singular vectors, but a rotation of the vectors partitions the large majority of the signal into only ®ve singular vectors. Fitting the timedependent vectors to a sum of simple exponentials suggests that a chemical kinetic mechanism can describe the timedependent structural data. Procedures for the effective SVD analysis of experimental time-resolved crystallographic data have been established and emphasize the necessity for minimizing systematic errors by modi®cation of the datacollection protocol.

The electronic relaxation and isomerization mechanism of trans-azobenzene after the S 2 (ππ*) r S 0 photoexcitation were investigated in solution by steady-state and femtosecond time-resolved fluorescence spectroscopy. In the steady-state fluorescence spectrum, two bands were observed with their peaks at ∼390 nm (∼25 750 cm -1 ) and ∼665 nm (∼15 000 cm -1 ). These fluorescence bands showed good mirror images of the S 2 (ππ*) r S 0 and S 1 (nπ*) r S 0 absorption bands, so that they were assigned to the fluorescence from the S 2 (ππ*) and S 1 (nπ*) states having "planar" structures. The lifetimes of the S 2 and S 1 states were determined as ∼110 fs (S 2 ) and ∼500 fs (S 1 ) by time-resolved measurements. The quantum yield of the S 2 f S 1 electronic relaxation was evaluated by comparing the intensity of the S 2 and S 1 fluorescence, and it was found to be almost unity. This implies that almost all molecules photoexcited to the S 2 (ππ*) state are relaxed to the "planar" S 1 (nπ*) state. The present fluorescence data clarified that the isomerization following S 2 (ππ*) photoexcitation takes place after the S 2 f planar S 1 electronic relaxation and that the rotational isomerization pathway starting directly from the S 2 (ππ*) state does not exist. It was thus indicated that the isomerization mechanism of azobenzene is the inversion isomerization occurring in the S 1 state, regardless of difference in initial photoexcitation. The relaxation pathways in the S 1 state were also discussed on the basis of spectroscopic and photochemical data.

A new technique of color detection based on time-resolved charge collection during transient response measurement using a hydrogenated amorphous silicon detector has been proposed and experimentally demonstrated. The method based on the material intrinsic wavelength-filtering properties is implemented in color sensing by transient response of a detector to a combination of basic colors light and voltage pulse. Resulting transient responses under different illumination and bias voltage conditions confirm considerable accuracy of proposed method. In particular, the best agreement of measured and calculated transient response curves has been obtained by using the spectral response of all three primary colors in calculations. It was observed that the presence of blue light drastically reduces the transient response to a chromatic light illumination. Finally, a voltage pulse influence to the basic colors transient response was found.

A diagrammatic perturbative approach is adopted to derive the expression for the third-order polarization, p(8) which originates the anti-Stokes emission in a time-resolved CARS experiment. The various contributions to P(3) are calculated assuming that laser fields are off-resonance with respect to any electronic level of material system. The resonant part of the polarization consists of two terms, in which the roles of the pump and probe pulses at frequency oJ1 are exchanged. The global expression allows the direct calculation of the signal time profile in a transient CARS experiment once a model is assumed for the laser pulse shape.

It was shown recently on the basis of DFT calculations (N. P. Gritsan and E. A. Pritchina, Mendeleev Commun., 2001, 11, 94) that the singlet states of aroylnitrenes undergo tremendous stabilization due to an extra N-O bonding interaction. To test experimentally the multiplicity and the structure of the lowest state of benzoylnitrenes we performed a study of their photochemistry in Ar matrices at 12 K. Formation of two species was observed on irradiation of benzoyl azide (1b) and its 4-acetyl derivative (1c). One of these species has an IR spectrum, which is consistent with that of isocyanate (2b,c). The IR and UV spectra of the second intermediate are in very good agreement with the calculated spectra of the singlet species (3b,c), whose structure is intermediate between that of a carbonylnitrene and an oxazirene. We further examined the photochemistry of benzoyl azide in solution at ambient temperatures by nanosecond time-resolved IR methods and obtained additional evidence for the singlet ground state of benzoylnitrene as well as insight into its reactivity in acetonitrile, cyclohexane, and dichloromethane. The above experiments were accompanied by quantum chemical calculations which included also a thorough investigation of the parent species, formylnitrene, at different levels of theory.

Pulsed optically stimulated luminescence measured from quartz under certain combinations of annealing and measurement temperature can be resolved into multiple lifetime components. We have studied the influence on these lifetime components of annealing temperature up to 900 C, beta irradiation dose as high as 1700 Gy, and temperature of stimulation up to 200 C. Although the time-resolved spectra from which the lifetimes are determined may be measured without heating between irradiation and measurement, a necessary protocol in steady-state optical stimulation applications, studies reported in this paper have been augmented with investigations on the effect of temperature and duration of preheating on the principal and secondary lifetimes. Luminescence spectra for measurements up to 200 Gy consist of a single lifetime but two components occur thereafter, both are independent of radiation dose. The principal lifetime of w35 ms is not affected by temperature of anneal whereas the secondary lifetime increases from 9 to 18 ms when the annealing temperature is changed from 600 to 900 C. The dependence of both the principal and secondary luminescence lifetimes on measurement temperature can be explained by thermal quenching of the associated luminescence. However, the influence of irradiation, preheating, and annealing on the lifetimes can be explained with reference to an energy band scheme in which the primary mechanism in the processes leading up to luminescence emission is the change in concentration of holes at various luminescence centres caused by annealing.

A flow visualisation study was performed to investigate a periodic flow instability in a bifurcating duct within the tip of the flares at the Shell refinery in Clyde, NSW, to verify the trigger of a combustion-driven oscillation proposed in Part A of this study, and to identify its features. The model study assessed only the flow instability, uncoupled from the

Dynamic evaluation of lower limb arteries using the ECTRICKS MRI technique J Radiol 2005;86:49-59 Purpose. This article presents a new MR angiography technique, Elliptic Centric Time Resolved Imaging on Contrast KineticS (ECTRICKS), which allows dynamic evaluation of the lower limb arteries opacification when venous return prevents adequate diagnostic evaluation of calf arteries. Patient and methods. Comparative examination of the calf arteries in 59 patients using a standard MRA technique and the ECTRICKS technique. Results. A total of 33 examinations could be compared. Additional arteries could be evaluated in 10% of our patient population using the ECTRICKS technique, and in 20% for our diabetic population. Arterial evaluation was better with the ECTRICKS technique in patients with diabetes and venous insufficiency. Conclusion. ECTRICKS is a good complementary sequence for evaluating the calf arteries in patients with diabetes or early venous return. Objectif. Cette étude présente une nouvelle technique d'angioIRM, Elliptic Centric Time Resolved Imaging on Contrast KineticS, qui permet l'étude dynamique de l'opacification des artères des membres inférieurs quand le retour veineux masque les artères jambières et gène le diagnostic. Patient et méthode. Examen comparatif des artères jambières chez 59 patients avec l'angio-IRM standard et avec ECTRICKS. Résultats. Trente trois examens ont permis une étude comparative. Dix pour cents d'artères supplémentaires étaient analysables en ECTRICKS, 20 % pour la population diabétique. Les artères étaient mieux analysables en ECTRICKS dans la population diabetique et en cas d'insuffisance veineuse. Conclusion. ECTRICKS est une séquence complémentaire pour l'étude des artères des membres inférieurs en cas de retour veineux gênant dans la population diabétique ou en cas de retour veineux jambier précoce.

Three different sites of the Cr 3ϩ ions in the fluoride perovskite KMgF 3 have been identified by absorption, selective optical excitation, and time-resolved emission spectroscopy of the 4 T 2 ↔ 4 A 2 transition. Highpressure measurements showing the crossover from low-crystal field to high-crystal field, allows us to situate the Dq/B values of the different chromium sites clearly below 2.3. The different spin-orbit components associated with the zero-phonon lines of the 4 T 2 ↔ 4 A 2 transition of each type of site are clearly shown on the optical spectra and identified by group-theory analysis. The decay profiles of the 4 T 2 level are exponential and the lifetimes at 15 K are 903, 919, and 473 s for the cubic, quadratic, and trigonal sites, respectively. The trigonal center which presents the lowest energy levels was peculiarly studied. Emission and excitation spectra are compared; their evolution versus temperature is followed and explained by the thermal population of the different spin-orbit sublevels. The phonon sideband of the trigonal site is compared with our previous lattice dynamic studies of the pure compound. The different peaks of the emission broadband are described in terms of phonons of the matrix and normal modes of the ͓CrF 6 ͔ 3Ϫ complex. ͓S0163-1829͑97͒03430-9͔

The transformation of mesomorphic phase to a-monoclinic crystal phase in quenched isotactic polypropylene has been investigated by TEM, DSC and time-resolved SAXS and WAXD methods. It is found that even though the initial appearance of the cluster structure in mesomorphic i-PP seems to support the model of a multi-step process for polymer crystallization, results indicate that the transformation is not spontaneous. In the cluster domains, a signi®cant fraction of chain segments must undergo a reorganization process in order to establish the correct registration of helical hands for crystallization. In addition, a fraction of the ordered chains with correct registration of helical hands should also serve as primary nuclei to initiate crystallization. However, the entire cluster domains should not be considered as precursors'. The growth process via secondary nucleation eventually transforms the cluster structure to the lamellar structure. q

The photoluminescence properties of type I sol-gel synthesized Rhodamine 6G doped silica samples were investigated in the 1.5 · 10 À4 -1.5 · 10 À3 mol/l range of concentration by means of steady-state and time resolved photoluminescence measurements. The emission peak red shifts as the concentration increases. For a fixed concentration, the peak position also red shifts as the excitation wavelength decreases. The observed spectroscopic features, including excitation of photoluminescence and decay time, indicate the formation of fluorescent dimers.

We present a detailed study of the evolution of the size, structure and stability of casein micelles upon acidification of skim milk typically applied in yogurt-making processes using a combination of time-resolved light and small-angle X-ray scattering experiments. While most of the available light scattering studies on casein acidification have been restricted to transparent and therefore highly diluted samples, we now profit from a newly developed multiangle 3D light scattering instrument, which allows for time-resolved measurements in highly turbid samples. Our experiments clearly demonstrate the presence of two parallel pH-dependent processes, micellar reassembly and aggregation. Using a systematic investigation of the effect of casein concentration, acidification rate, and ionic strength, we are able to decouple these two processes and obtain detailed information about the pH-induced restructuration of the casein micelle structure that occurs prior to destabilization. Moreover, our experiments also unambiguously demonstrate that these micellar reassembly processes are highly concentration dependent, and that typical light scattering studies conducted under highly diluted conditions are resulting in findings that may not be relevant for the situation encountered in industrial processes at higher concentrations. Experiments conducted with covalently cross-linked micelles, where the pH-induced reassembly has been suppressed, further confirm our findings.

We present a novel technique for wide dynamic range optical investigations. It is based on a fast-gated silicon singlephoton avalanche diode (SPAD) in time-correlated single-photon counting (TCSPC) setup. The SPAD is gated-ON and OFF in 500 ps so as to detect photons only within a given time interval. This technique is particularly useful in applications where a large amount of unnecessary photons precede or follow the optical signal to be detected, such as in time-resolved near infrared (NIR) spectroscopy, optical mammography, and optical molecular imaging. In particular, in time-resolved reflectance spectroscopy, it is desirable to minimize the source-detector separation to improve system performance. This leads to the saturation of the detection electronics because of the huge amount of "early" photons back scattered by superficial layers. Our setup is able to reject these photons and detect only "late" photons from the sample, thus allowing an increase in the dynamic range and the injected power. We acquired diffusive curves of two phantoms with 95 ps time resolution and 10 7 dynamic range with a measurement time three orders of magnitude shorter than what is currently possible with a standard TCPSC setup.

Femtosecond time-resolved photoelectron spectroscopy is emerging as a new technique for investigating polyatomic excited state dynamics. Due to the sensitivity of photoelectron spectroscopy to both electronic configurations and vibrational dynamics, it is well suited to the study of ultrafast non-adiabatic processes such as internal conversion, often occurring on sub-picosecond time scales. We discuss technical requirements for such experiments, including laser systems, energy and angle resolved photoelectron spectrometers and new detectors for coincidence experiments. We illustrate these methods with examples from diatomic wavepacket dynamics and ultrafast non-adiabatic processes in polyatomic molecules.

We report the vibrational and orientational dynamics of water molecules in isotopically diluted NaOH and NaOD solutions using polarization-resolved femtosecond vibrational spectroscopy and terahertz time-domain dielectric relaxation measurements. We observe a speed-up of the vibrational relaxation of the O-D stretching vibration of HDO molecules outside the first hydration shell of OH − from 1.7 ± 0.2 ps for neat water to 1.0 ± 0.2 ps for a solution of 5 M NaOH in HDO:H 2 O. For the O-H vibration of HDO molecules outside the first hydration shell of OD − , we observe a similar speed-up from 750 ± 50 fs to 600 ± 50 fs for a solution of 6 M NaOD in HDO:D 2 O. The acceleration of the decay is assigned to fluctuations in the energy levels of the HDO molecules due to charge transfer events and charge fluctuations. The reorientation dynamics of water molecules outside the first hydration shell are observed to show the same time constant of 2.5 ± 0.2 ps as in bulk liquid water, indicating that there is no long range effect of the hydroxide ion on the hydrogen-bond structure of liquid water. The terahertz dielectric relaxation experiments show that the transfer of the hydroxide ion through liquid water involves the simultaneous motion of ∼7 surrounding water molecules, considerably less than previously reported for the proton.

. We examined the time-resolved and steady-state fluorescence quenching of N-acetyl-L-tryptophanamide NATA by acrylamide and iodide, over a range of viscosities in propylene glycol. The quenching of NATA by acrylamide and iodide results in heterogeneity of the intensity decay which increases with the quencher concentration. We attribute the complex decays of NATA to transient effects in diffusion and the nature of the fluorophore᎐quencher Ž . interaction. These data were compared using the phenomenological radiation boundary condition RBC and Ž . distance-dependent quenching DDQ models for collisional quenching. We used global analysis of the time-resolved frequency-domain and steady-state data to select between the models. Consideration of both the frequency-domain and steady state data demonstrate that the quenching rate depends exponentially on the fluorophore᎐quencher distance, indicating the validity of the DDQ model. The rate constants for acrylamide and iodide quenching, at the 13 y1 9 y 1 constant distance of 5 A, were found to be near 10 s and 10 s , respectively. These rates reflect electron transfer and exchange interactions as the probable quenching mechanisms, respectively. ᮊ

We report the first experimental detailed study of laser induced surface melting on the nanoscale time scale of amorphous silicon carbide layers produced by ion implantation. Time-resolved reflectivity has been used to observe the fast liquid-solid-liquid transition features, and transmission electron microscopy (TEM) was used in order to study the structure resulting after the fast solidification following the laser induced melting. By means of the evaluation of the laser fluences required to induce melting of amorphous layers of different thickness on top of a crystalline substrate, we evaluated the thermal diffusion coefficient and the melting point of the amorphous material which occurred much lower than for crystalline material. Moreover, we give evidence of amorphous-to-crystal transitions occurring in the solid phase on the nanosecond time scale, for laser irradiation at fluences below the melting threshold. A quite different crystalline structure is observed for crystallization from the liquid phase than from the solid phase.